It is known to maintain the flow of fluid in a compressor above a minimum flow level, and a controller has been associated with the compressor control system imposing a control setpoint to this effect. As shown in U.S. Pat. No. 3,380,650, the inlet valve is controlled downward only to a minimum positioning level, in order to avoid a nonsteady situation which would cause a surge. This limit has been called the surge point. It is also known from this same patent to recognize such minimum positioning of the inlet valve by sensing a minimum horse power from the motor driving the compressor. Such minimum positioning, however, is not maintained by the system over the long term. It is now proposed, when controlling the bypass valve, to automatically maintain the inlet valve at its minimum position, while using the current of the motor driving the compressor as an indicator.
In this regard, it is known from U.S. Pat. No. 3,535,053 to use the current in the motor driving a compressor for establishing a limit overriding any normal further adjustment of the vane of the compressor. From U.S. Pat. No. 4,080,110, it is known to regulate compressor capacity in relation to the current in the motor driving the compressor, so as to maintain a predetermined motor input current. U.S. Pat. No. 4,519,748 shows control of the slide valve of a helical screw type compressor while continuously seeking the position at which the compressor drive motor current is at a minimum U.S. Pat. No. 4,462,217 discloses, upon changes in the dynamic pressure of a propulsion system driven by an electric motor, how a constant flow of fluid is maintained.
U.S. Pat. No. 3,778,695 discloses the use of a minimum value of the current of the motor driving a compressor as a limit for transferring control of the inlet valve to control of the bypass valve of the compressor.
A surge is known to occur in a centrifugal compressor when the back pressure of the load becomes greater than the compressor pressure. It is known to prevent such occurrence by using a blow-off, or bypass valve, to vent the compressor when the flow falls below a preset minimum. See for instance U.S. Pat. No. 3,863,110.
It is acknowledged as known in U.S. Pat. No. 4,191,511 to operate a compressor at a preselected constant speed with the output being regulated over a relatively narrow range to provide compressed gas to a process at a rate sufficient to provide the maximum needed compressed gas while using a vent, or relief valve for increased range of operation.
While it is recognized that for continuous operation a minimum flow rate should at all time be maintained in order for the compressor to be ready to supply the new demand following a fall of the demand, such minimum flow rate cannot be relied upon without attending to its determination from time to time. The object of the present invention is to provide a reliable minimum flow rate at all times without impeding readiness for further processing nor the establishing of a steady state at low ebb pending the return of the normal demand. Such an approach insures long term and continuous operation of a compressor system under extreme load demands without any risk of a surge.